Ad Hominem

(Attacking the person): This fallacy occurs when, instead of addressing someone's argument or position, you irrelevantly attack the person or some aspect of the person who is making the argument. The fallacious attack can also be direct to membership in a group or institution.

Examples:

Student: Hey, Professor Moore, we shouldn't have to read this book by Freud. Everyone knows he used cocaine.

Socrates' arguments about human excellence are rubbish. What could a man as ugly as he know about human excellence.

Yeah, I think everyone's opinion counts on moral matters like that, but that Lila sleeps around with anything. I know of at least one marriage she's broken up, so why should her opinion count on anything, much less morality?

Of course Marx' theories about the ideal society are bunk. The guy spent all his time in the library.

We cannot approve of this recycling idea. It was thought of by a bunch of hippie communist weirdos.

There's no reason to take seriously Nietzsche's ideas about the Superman. Weak and sickly all his short life, of course he found this concept captivating. In psychology, we call this compensation.

I was assigned a personal trainer at the Rec, and he gave me a new workout program. But I don't have any confidence in his expertise, since he has obvious trouble controlling his own appetite.

No, I will not reply. I see no need to defend my views against the objections of ignoramuses.